Compass Primer - Revision history

Mparadis: /* Declination */

2017-06-07T17:02:04Z

Declination

← Older revision		Revision as of 17:02, 7 June 2017
Line 58:		Line 58:
	Over long periods of time changes in the Earth's magnetic core have an effect on the Earth's magnetic field. What this means is that the North Magnetic Pole shifts and is not the same as "true" North. The time it takes for magnetic North to shift significantly is very long so for all intents and purposes the magnetic North pole is stationary, it is just important to note that it is not in the same place as the actual North pole. Since compasses measure direction as a function of magnetic field strength, the direction a compass tells you is North is in fact magnetic North. As you might imagine the amount that magnetic North and true North differ is a function of your location. It is technically possible for your compass to point in the exact opposite direction from North if you are directly between the North pole and the magnetic North pole. The most common way to get declination is from a map that has isogonic lines. Isogonic lines are lines of equal declination similar to contour lines on topographic maps that show elevation. The lines will give a reasonable approximation of the declination for your region. For example, take a look at the map below and try to determine what the declination is in your city:		Over long periods of time changes in the Earth's magnetic core have an effect on the Earth's magnetic field. What this means is that the North Magnetic Pole shifts and is not the same as "true" North. The time it takes for magnetic North to shift significantly is very long so for all intents and purposes the magnetic North pole is stationary, it is just important to note that it is not in the same place as the actual North pole. Since compasses measure direction as a function of magnetic field strength, the direction a compass tells you is North is in fact magnetic North. As you might imagine the amount that magnetic North and true North differ is a function of your location. It is technically possible for your compass to point in the exact opposite direction from North if you are directly between the North pole and the magnetic North pole. The most common way to get declination is from a map that has isogonic lines. Isogonic lines are lines of equal declination similar to contour lines on topographic maps that show elevation. The lines will give a reasonable approximation of the declination for your region. For example, take a look at the map below and try to determine what the declination is in your city:

	[[File: declination_small.jpg\|link={{SERVER}}/~~wiki~~/images/f/f3/Declination.jpg]]		[[File: declination_small.jpg\|link={{SERVER}}/docs21/images/f/f3/Declination.jpg]]

	For the purposes of calculation assume that East declination is positive and West is negative. This means that you need to subtract the declination from your heading when your declination is to the East and vice versa when it is West.		For the purposes of calculation assume that East declination is positive and West is negative. This means that you need to subtract the declination from your heading when your declination is to the East and vice versa when it is West.

Burley: /* Calculating Heading from Magnetometer Data */

2014-10-14T15:19:09Z

Calculating Heading from Magnetometer Data

← Older revision		Revision as of 15:19, 14 October 2014
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	//Pitch Angle - about axis 1		//Pitch Angle - about axis 1
	// tan(pitch angle) = -gx / (gy * sin(roll angle) * gz * cos(roll angle))		// tan(pitch angle) = -gx / ((gy * sin(roll angle)) + (gz * cos(roll angle)))
	// Pitch angle range is (-90 - 90) degrees		// Pitch angle range is (-90 - 90) degrees
	double pitchAngle = Math.Atan(		double pitchAngle = Math.Atan(

Burley: /* Calibration */

2013-06-20T14:20:30Z

Calibration

← Older revision		Revision as of 14:20, 20 June 2013
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	Calibration is ''required'' for any applications needing more accuracy than a simple North, East, South, or West. If you have already gathered compass data, you can retroactively calibrate that data by applying the same calibration formula to each data point. You can find these formulae by looking at the source code of phidgetspatial.c in the [[OS_-_Linux#Quick_Downloads\|Linux source code]].		Calibration is ''required'' for any applications needing more accuracy than a simple North, East, South, or West. If you have already gathered compass data, you can retroactively calibrate that data by applying the same calibration formula to each data point. You can find these formulae by looking at the source code of phidgetspatial.c in the [[OS_-_Linux#Quick_Downloads\|Linux source code]].

	For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]].		For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]]. The newer, 1042 and 1044s also require calibration and you can find a guide for calibrating them in their respective User Guides.

	==Interference==		==Interference==

Mparadis: /* Calibration */

2013-03-21T15:40:39Z

Calibration

← Older revision		Revision as of 15:40, 21 March 2013
Line 17:		Line 17:
	Calibration involves setting the compass up to handle the magnetic field characteristics in your particular region of the world as well as disturbances caused by a more localized magnetic fields such as speaker magnets. To calibrate effectively you will need to determine the Earth's magnetic field strength at your location. This data is available on line in the form of [http://www.ngdc.noaa.gov/geomag-web/#igrfwmm magnetic field calculators] all you have to do is enter in your zip or postal code.		Calibration involves setting the compass up to handle the magnetic field characteristics in your particular region of the world as well as disturbances caused by a more localized magnetic fields such as speaker magnets. To calibrate effectively you will need to determine the Earth's magnetic field strength at your location. This data is available on line in the form of [http://www.ngdc.noaa.gov/geomag-web/#igrfwmm magnetic field calculators] all you have to do is enter in your zip or postal code.

	Calibration is ''required'' for any applications needing more accuracy than a simple North, East, South, or West. If you have already gathered compass data, you can retroactively calibrate that data by applying the same calibration formula to each data point.		Calibration is ''required'' for any applications needing more accuracy than a simple North, East, South, or West. If you have already gathered compass data, you can retroactively calibrate that data by applying the same calibration formula to each data point. You can find these formulae by looking at the source code of phidgetspatial.c in the [[OS_-_Linux#Quick_Downloads\|Linux source code]].

	For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]].		For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]].

Mparadis: /* Calibration */

2013-03-07T17:23:24Z

Calibration

← Older revision		Revision as of 17:23, 7 March 2013
Line 17:		Line 17:
	Calibration involves setting the compass up to handle the magnetic field characteristics in your particular region of the world as well as disturbances caused by a more localized magnetic fields such as speaker magnets. To calibrate effectively you will need to determine the Earth's magnetic field strength at your location. This data is available on line in the form of [http://www.ngdc.noaa.gov/geomag-web/#igrfwmm magnetic field calculators] all you have to do is enter in your zip or postal code.		Calibration involves setting the compass up to handle the magnetic field characteristics in your particular region of the world as well as disturbances caused by a more localized magnetic fields such as speaker magnets. To calibrate effectively you will need to determine the Earth's magnetic field strength at your location. This data is available on line in the form of [http://www.ngdc.noaa.gov/geomag-web/#igrfwmm magnetic field calculators] all you have to do is enter in your zip or postal code.

	Calibration is ''required'' for any applications needing more accuracy than North, East, South, West.		Calibration is ''required'' for any applications needing more accuracy than a simple North, East, South, or West. If you have already gathered compass data, you can retroactively calibrate that data by applying the same calibration formula to each data point.

	For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]].		For more information on calibrating the compass on the [[1056 User Guide\|1056]] refer to our [[1056 User Guide#Magnetic Error Correction (Calibration)\|calibration guide]].

Burley: /* Declination */

2012-09-06T18:54:14Z

Declination

← Older revision		Revision as of 18:54, 6 September 2012
Line 58:		Line 58:
	Over long periods of time changes in the Earth's magnetic core have an effect on the Earth's magnetic field. What this means is that the North Magnetic Pole shifts and is not the same as "true" North. The time it takes for magnetic North to shift significantly is very long so for all intents and purposes the magnetic North pole is stationary, it is just important to note that it is not in the same place as the actual North pole. Since compasses measure direction as a function of magnetic field strength, the direction a compass tells you is North is in fact magnetic North. As you might imagine the amount that magnetic North and true North differ is a function of your location. It is technically possible for your compass to point in the exact opposite direction from North if you are directly between the North pole and the magnetic North pole. The most common way to get declination is from a map that has isogonic lines. Isogonic lines are lines of equal declination similar to contour lines on topographic maps that show elevation. The lines will give a reasonable approximation of the declination for your region. For example, take a look at the map below and try to determine what the declination is in your city:		Over long periods of time changes in the Earth's magnetic core have an effect on the Earth's magnetic field. What this means is that the North Magnetic Pole shifts and is not the same as "true" North. The time it takes for magnetic North to shift significantly is very long so for all intents and purposes the magnetic North pole is stationary, it is just important to note that it is not in the same place as the actual North pole. Since compasses measure direction as a function of magnetic field strength, the direction a compass tells you is North is in fact magnetic North. As you might imagine the amount that magnetic North and true North differ is a function of your location. It is technically possible for your compass to point in the exact opposite direction from North if you are directly between the North pole and the magnetic North pole. The most common way to get declination is from a map that has isogonic lines. Isogonic lines are lines of equal declination similar to contour lines on topographic maps that show elevation. The lines will give a reasonable approximation of the declination for your region. For example, take a look at the map below and try to determine what the declination is in your city:

	[[File: declination_small.jpg\|link=~~http://newsite.int.phidgets.com~~/wiki/images/f/f3/Declination.jpg]]		[[File: declination_small.jpg\|link={{SERVER}}/wiki/images/f/f3/Declination.jpg]]

	For the purposes of calculation assume that East declination is positive and West is negative. This means that you need to subtract the declination from your heading when your declination is to the East and vice versa when it is West.		For the purposes of calculation assume that East declination is positive and West is negative. This means that you need to subtract the declination from your heading when your declination is to the East and vice versa when it is West.

Patrick at 16:55, 15 August 2012

2012-08-15T16:55:58Z

← Older revision		Revision as of 16:55, 15 August 2012
Line 108:		Line 108:
	// tan(pitch angle) = -gx / (gy * sin(roll angle) * gz * cos(roll angle))		// tan(pitch angle) = -gx / (gy * sin(roll angle) * gz * cos(roll angle))
	// Pitch angle range is (-90 - 90) degrees		// Pitch angle range is (-90 - 90) degrees
	double pitchAngle = Math.Atan(-gravity[0] / (gravity[1] * Math.Sin(rollAngle) + gravity[2] * Math.Cos(rollAngle)));		double pitchAngle = Math.Atan(
			-gravity[0] / ((gravity[1] * Math.Sin(rollAngle)) + (gravity[2] * Math.Cos(rollAngle)))
			);

	//Yaw Angle - about axis 2		//Yaw Angle - about axis 2
Line 116:		Line 118:
	//		//
	// Yaw angle == 0 degrees when axis 0 is pointing at magnetic north		// Yaw angle == 0 degrees when axis 0 is pointing at magnetic north
	double yawAngle = Math.Atan2(magField[2] * Math.Sin(rollAngle) - magField[1] * Math.Cos(rollAngle),		double yawAngle = Math.Atan2(
	magField[0] * Math.Cos(pitchAngle) + magField[1] * Math.Sin(pitchAngle) * Math.Sin(rollAngle) + magField[2] * Math.Sin(pitchAngle) * Math.Cos(rollAngle));		(magField[2] * Math.Sin(rollAngle))
			- (magField[1] * Math.Cos(rollAngle))
			,
			(magField[0] * Math.Cos(pitchAngle))
			+ (magField[1] * Math.Sin(pitchAngle) * Math.Sin(rollAngle))
			+ (magField[2] * Math.Sin(pitchAngle) * Math.Cos(rollAngle))
			);

	double[] angles = {rollAngle, pitchAngle, yawAngle};		double[] angles = {rollAngle, pitchAngle, yawAngle};

Mparadis: /* What are likely sources of interference and how are they handled? */

2012-08-02T18:14:22Z

What are likely sources of interference and how are they handled?

← Older revision		Revision as of 18:14, 2 August 2012
Line 25:		Line 25:

	===What are likely sources of interference and how are they handled?===		===What are likely sources of interference and how are they handled?===
	[[image:nodistortion.gif\|thumb\|310px\|Approximation of 2 axis magnetic field data with 0 distortion.]]		[[image:nodistortion.gif\|thumb\|310px\|link=\|Approximation of 2 axis magnetic field data with 0 distortion.<br>[[Media:nodistortion.gif\|Full-Sized Image]]]]
	There are 4 main categories of interference. Some are easy to get rid of via [[Compass Primer#Calibration\|calibration]], others are not easy or even impossible to get rid of.		There are 4 main categories of interference. Some are easy to get rid of via [[Compass Primer#Calibration\|calibration]], others are not easy or even impossible to get rid of.

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	<br clear = "all">		<br clear = "all">
	===="Hard Iron" Interference====		===="Hard Iron" Interference====
	[[image:harddistortion.gif\|thumb\|310px\|Approximation of magnetic field with hard iron distortion]]		[[image:harddistortion.gif\|thumb\|link=\|310px\|Approximation of magnetic field with hard iron distortion.<br>[[Media:harddistortion.gif\|Full-Sized Image]]]]
	So called ''hard iron'' interference is caused by static magnetic fields associated with the vehicle. This can include any minor (or major) magnetism in the metal chassis or frame of the vehicle, any actual magnets attached such as speakers etc... This interference pattern is unique to the vehicle but is constant. If you have your compass in an enclosure that is held together with metal screws even these relatively small amounts of ferromagnetic material can cause issues. If we consider the magnetic data circle, hard iron interference has the effect of x/y(/z in the case of 3 axes) shifting the entire circle away from the origin by some amount. The amount is dependent on any number of different factors and can be very large. The important part is that this shift is the same for all points in time so it can be calibrated out very easily with a numeric offset which is taken care of by the [[Compass Primer#Calibration\|calibration process]].		So called ''hard iron'' interference is caused by static magnetic fields associated with the vehicle. This can include any minor (or major) magnetism in the metal chassis or frame of the vehicle, any actual magnets attached such as speakers etc... This interference pattern is unique to the vehicle but is constant. If you have your compass in an enclosure that is held together with metal screws even these relatively small amounts of ferromagnetic material can cause issues. If we consider the magnetic data circle, hard iron interference has the effect of x/y(/z in the case of 3 axes) shifting the entire circle away from the origin by some amount. The amount is dependent on any number of different factors and can be very large. The important part is that this shift is the same for all points in time so it can be calibrated out very easily with a numeric offset which is taken care of by the [[Compass Primer#Calibration\|calibration process]].
	<br clear = "all">		<br clear = "all">

	===="Soft Iron" Interference====		===="Soft Iron" Interference====
	{\|~~align~~ = ~~"right"~~
	\|[[image:Magnetic_distortion.jpg\|thumb\|~~310px~~\|Example of soft iron distortion caused by an ''external'' object.~~]]\|\|~~[[~~image~~:~~softdistortion~~.~~gif~~\|~~thumb\|310px\|Approximation of magnetic field with soft iron distortion~~]]
	\|}
			[[image:softdistortion.gif\|link=\|thumb\|310px\|Approximation of magnetic field with soft iron distortion.<br>[[Media:softdistortion.gif\|Full-Sized Image]]]]
			[[image:Magnetic_distortion.jpg\|link=\|thumb\|left\|250px\|Example of soft iron distortion caused by an ''external'' object.<br>[[Media:Magnetic_distortion.jpg\|Full-Sized Image]]]]

	''Soft iron'' interference is caused by distortion of the Earth's magnetic field due to materials in the vehicle's construction. Think of it like electricity, the magnetic field is looking for the easiest path to get to where it is going. Since magnetic fields can flow more easily through ferromagnetic materials than air, more of the field will flow through the ferromagnetic material than you would expect if it were just air. This distortion effect causes the magnetic field lines to be bent sometimes quite a bit. Note that unlike hard iron interference which is the result of materials which actually have a magnetic field of their own, soft iron interference is caused by non-magnetic materials distorting the Earth's magnetic field. This type of interference has a squishing effect on the magnetic data circle turning it into more of an ellipsoid shape. The distortion in this case depends on the direction that the compass is facing. Because of this the distortion cannot be [[Compass Primer#Calibration\|calibrated]] out with a simple offset, more complicated math will still let the compass account for this type of interference though.		''Soft iron'' interference is caused by distortion of the Earth's magnetic field due to materials in the vehicle's construction. Think of it like electricity, the magnetic field is looking for the easiest path to get to where it is going. Since magnetic fields can flow more easily through ferromagnetic materials than air, more of the field will flow through the ferromagnetic material than you would expect if it were just air. This distortion effect causes the magnetic field lines to be bent sometimes quite a bit. Note that unlike hard iron interference which is the result of materials which actually have a magnetic field of their own, soft iron interference is caused by non-magnetic materials distorting the Earth's magnetic field. This type of interference has a squishing effect on the magnetic data circle turning it into more of an ellipsoid shape. The distortion in this case depends on the direction that the compass is facing. Because of this the distortion cannot be [[Compass Primer#Calibration\|calibrated]] out with a simple offset, more complicated math will still let the compass account for this type of interference though.

Mparadis: /* Calculating Heading from Magnetometer Data */

2012-08-02T18:05:37Z

Calculating Heading from Magnetometer Data

← Older revision		Revision as of 18:05, 2 August 2012
Line 73:		Line 73:

	==Calculating Heading from Magnetometer Data==		==Calculating Heading from Magnetometer Data==
	The three axis magnetometer data can be used, along with pitch and roll angles calculated from the [[Accelerometer Primer\|\|accelerometer]], to find a magnetic bearing - ie. as a compass. In order for the compass to be accurate at all, [[#Calibration\|calibration]] must have been done to factor out hard and soft iron offsets.		The three axis magnetometer data can be used, along with pitch and roll angles calculated from the [[Accelerometer Primer\|accelerometer]], to find a magnetic bearing - ie. as a compass. In order for the compass to be accurate at all, [[#Calibration\|calibration]] must have been done to factor out hard and soft iron offsets.

	The magnetic field is represented as a vector. If the spatial is completely level with respect to gravity, then we can calculate magnetic bearing easily by looking at the x and y components of the vector, which simply represents an angle between 0 and 360 degrees on the unit circle.		The magnetic field is represented as a vector. If the spatial is completely level with respect to gravity, then we can calculate magnetic bearing easily by looking at the x and y components of the vector, which simply represents an angle between 0 and 360 degrees on the unit circle.

Burley at 17:43, 2 August 2012

2012-08-02T17:43:23Z

← Older revision		Revision as of 17:43, 2 August 2012
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	[[Category:Primer]]		[[Category:Primer]]
	~~'''Under Construction'''~~
	~~:*Needs non-licensed images for interference section~~

	__TOC__		__TOC__